The present invention relates to a solar energy collector having an improved absorbing layer of a coated foil adapted to decrease the cost of manufacturing the collector while providing still other improvements.
Realization that the fossil fuel supply of the world is finite, and may be rapidly depleted at the present rate of national energy consumption, has led to a search for substitute energy resources. Use of solar radiation is one possibility for providing clean and reliable energy.
Solar energy is an extensive, constant energy source whose economic feasibility depends on efficient collection, retention, and utilization. The efficiency of some solar collecting systems has been low due to excessive heat losses. One area in which improvement has been sought is in solar selective absorber coatings, that is, coatings which absorb energy particularly well in the solar spectrum while minimizing heat loss by radiation. For example, such coatings are designed to collect thermal energy from exposure to solar radiation and then transmit the collected energy through other media either to heat or cool homes and buildings through heat exchangers.
In general, when radiant energy from the sun impinges on a cooler object, part of the energy is reflected and lost and the balance either absorbed or transmitted away. The absorber energy may be re-radiated at a longer wavelength. Accordingly, a coating which absorbs in the range of solar radiation becomes heated, provided the surface does not re-radiate or emit most or all of the energy collected.
Solar radiation reaching the surface of the earth is almost entirely confined to wavelengths in the range of 0.3 to 2.5 microns. It is estimated that about 90% of solar radiation is at wavelengths of about 0.4 microns to about 1.5 microns. The amount of radiation above 2.5 microns is negligible. Solar energy selective coatings, therefore, are designed to differentiate in their absorption, reflection or transmission characteristics between wavelengths above about 2.5 microns and wavelengths below about 2.5 microns. Thus, solar energy can be collected at wavelengths below about 2.5 microns and the collected energy then transferred to useful application at wavelengths above about 2.5 microns.
This also means that for effective collection and retention, a solar collector should absorb strongly at wavelengths below about 2.5 microns. A coating which has a high absorptivity, usually termed alpha, in the solar spectrum but a low emissivity, epsilon, at the temperature at which the collector operates may be called a solar selective coating. Even though a high alpha to epsilon ratio is desirable, it is essential that the alpha value be near one to collect as much of the available energy as possible.
Solar selective coatings are an important way to increase the efficiency of solar energy collectors, primarily by maximizing the absorption of solar energy and minimizing the energy lost by radiation. However, solar selective coatings can still suffer from several shortcomings. For example, the preparation of such coatings involves vacuum evaporation techniques which are relatively expensive. Normally, the vacuum deposition is made directly onto parts of the solar energy collector. This requires thorough cleaning of the parts to insure a properly adherent coating. Moreover, "no-flow" conditions of a collector, that is, when a heat-absorbing medium is not being circulated through the collector, decrease the stability of prior solar energy absorptive coatings over a period of time.